The manufacture of semiconductor integrated circuits typically involves highly complex, time consuming and costly processes which, with continually narrower line width requirements, must be achieved with an ever increasing degree of precision. Within such processes the etching of semiconductor material (e.g., silicon) often entails the use of a chemical bath to which a patterned semiconductor material is exposed, so as to etch selectively certain portions of the surface of a wafer. In a typical chemical etch process, both the rate of etch and the selectivity of etch are parameters critical to the successful formation of an intended substrate geometry. For the fabrication of devices having a so-called surface "strap" it is desirable that an etchant be able to select between a silicon substrate and a p-doped pattern in the silicon. Thus, for fabricating a semiconductor device having a p-doped strap on a silicon surface, the silicon surface can be implanted or diffused with a p-type dopant, such as boron, in a pattern containing at least 2.times.10.sup.19 atoms/cm.sup.3 of boron in the appropriate configuration. If one then has an etchant that can distinguish between the p-doped silicon and the undoped silicon, one can dissolve a portion of the surface of the silicon and leave behind a raised pattern of p-doped straps.
It is known in the art to employ potassium hydroxide(KOH)-based etchants for anisotropically etching silicon, and various lower alkyl alcohols have been added to the KOH solution to control the etching process. Thus, Kragness et al. (U.S. Pat. No. 3,506,509) describes an etchant comprising KOH, n-propanol and sec-butanol which is said to exhibit a substantially level etch rate. Erdman and Schmidt (U.S. Pat. No. 3,738,881) disclose an etchant comprising potassium, sodium, cesium, rubidium or quaternary ammonium hydroxide and an aliphatic alcohol. They indicate that aliphatic alcohols, such as n-propyl, isopropyl, n-butyl, sec-butyl, tert-butyl, pentyl and hexyl alcohols, are more suitable passivating agents for their purposes than are aromatic alcohols. Other patents disclose etchants comprising (a) alkali hydroxides and (b) methanol, ethanol, n-propanol, ethylene glycol, ethylene diamine, and other amines. Hall et al. (U.S. Pat. No. 3,160,539) disclose a solution of catechol in hydrazine as a silicon-selective etchant, although the significant hazards of hydrazine and the rapid oxidative deterioration of catechol make it industrially unattractive. Our own U.S. Pat. No. 4,941,941 discloses an etching solution comprising an aromatic compound having at least two adjacent hydroxyl groups, an amine and water. Two good reviews of anisotropic etching have been published: E. Bassous Electrochemical Technology in Electronics ECS Symposium 1988, p. 123 and W. Kern RCA Review 39, 278-308 (1978) p. 292.
Of the known methods for controlled etching, the combination of KOH and isopropanol is probably the most commonly used at present. Like all the other etchant compositions of the art, it suffers from two major drawbacks: (1) large proportions of the alcohol component are necessary to achieve modest selectivity between doped and undoped silicon, and (2) the alcohol must be used in an industrial setting at temperatures above its flash point.
Thus, there is a need for an etchant composition and method that do not give rise to the safety hazards associated with using flammable solvents above their flash points.
There is a further need for a composition and method that are highly selective for silicon in the presence of p-doped silicon.